1. Field of the Invention
The invention relates generally to rotary drag bits for drilling subterranean formations and, more particularly, to rotary drag bits employing superabrasive backup cutters rotationally trailing superabrasive primary cutters on selected areas over the bit face.
2. State of the Art
So-called xe2x80x9cbackupxe2x80x9d cutters have been conventionally employed for some time on rotary drag bits employing superabrasive primary cutters in the form of polycrystalline diamond compacts, or PDC""s, the primary cutters being oriented with their superabrasive cutting faces oriented generally in the direction of intended bit rotation. Backup cutters are typically employed for drilling applications involving penetration of hard or abrasive subterranean formations. The use of backup cutters has proven to be a convenient technique for gaining more superabrasive volume bearing on the formation to extend the life of a bit and enhance its stability without the necessity of designing the bit with excess blades to carry more PDC""s, the presence of additional blades increasing the design complexity and fabrication cost of the bit as well as potentially compromising bit hydraulics due to reduced flow area over the bit face and less-than-optimum nozzle placement. However, conventional backup cutters are fairly aggressive, and their placement and orientation on a blade, in combination with associated primary cutters, may lead to balling of the blade area with formation material.
Various approaches have been taken to increasing the wear-resistance of rotary drag bits using hard or superabrasive structures on the bit face in addition to superabrasive cutters. For example, U.S. Pat. No. 4,554,986 to Jones discloses the use of xe2x80x9crelatively hardxe2x80x9d wear elements such as tungsten carbide or diamond on ridges rotationally leading an associated row of superabrasive cutters. U.S. Pat. Nos. 4,718,505 and 4,823,892 to Fuller disclose the use of so-called xe2x80x9cabrasion elementsxe2x80x9d trailing a primary cutting structure, the abrasion elements comprising superabrasive particles embedded in a stud trailing a preform synthetic diamond cutter or embedded in a stud carrying a preform synthetic diamond cutter. U.S. Pat. Nos. 4,889,017 and 4,991,670 to Fuller et al. disclose the use of so-called xe2x80x9csecondxe2x80x9d cutting structures carrying embedded superabrasive particles and rotationally trailing xe2x80x9cfirstxe2x80x9d cutters comprising preform synthetic diamond. U.S. Pat. No. 4,942,933 to Barr et al. discloses xe2x80x9cback-upxe2x80x9d assemblies comprising, for example, bosses of cemented tungsten carbide impregnated with natural diamonds and rotationally trailing other cutter assemblies. U.S. Pat. No. 5,186,268 to Clegg discloses the use of so-called xe2x80x9csecondary elementsxe2x80x9d rotationally trailing xe2x80x9cprimaryxe2x80x9d cutting elements and alternatively comprising superabrasive particles embedded in a stud, a single superabrasive body embedded in the outer tip of a stud, or a domed-end stud or xe2x80x9cbuttonxe2x80x9d over which is applied an outer layer of polycrystalline diamond. U.S. Pat. No. 5,222,566 to Taylor et al. depicts, but does not appear to discuss, structures rotationally trailing cutter assemblies carried on leading edges of blades on a bit. U.S. Pat. No. 5,244,039 to Newton et al. discloses the use of xe2x80x9csecondary elementsxe2x80x9d rotationally trailing primary cutting elements, the exposure of the secondary elements varying with distance from the nose portion of the bit face. U.S. Pat. No. 5,303,785 to Duke discloses the use of ribs carrying PDC cutting elements at rotationally leading ends thereof, the ribs carrying diamond or other ultra-hard segments embedded in the outwardly facing surfaces thereof and rotationally behind the PDC cutting elements. U.S. Pat. No. 5,595,252 to O""Hanlon discloses the alternative use of structures either rotationally trailing or leading preform cutting elements to control penetration of the latter into a formation being drilled.
Drill bits carrying conventional structures to reduce wear resistance fail to provide sufficient enhancement of the volume of superabrasive material in critical areas over the bit face, and are not effective in providing a dynamically stable cutting action due to their radial aggressiveness.
The present invention provides a radially unaggressive, tangentially efficient supplemental cutting element exhibiting a relatively large volume of superabrasive material for enhanced impact and wear resistance of an associated, more aggressive, differently oriented cutting element on the body of a rotary drag bit, as well as affording protection for the bit body and enhanced stability during drilling. The supplemental cutting element is configured and mounted on the bit body so as to minimize additional torque required to rotate the bit by providing a bearing surface under forces pushing the supplemental cutting element against the formation being drilled in a direction substantially perpendicular to the bit face profile at the location of the supplemental cutting element while affording the capability of cutting the formation being drilled with the superabrasive material of the supplemental cutting element in the direction of bit rotation should one or more associated primary cutting elements unduly wear or fail during drilling.
The present invention comprises a cutting assembly for use in rotary drag bits, such cutting assembly comprising, in one embodiment, a first, relatively more aggressive cutting element having a superabrasive table with a cutting face oriented generally in a direction of intended bit rotation, and a second, relatively less aggressive cutting element rotationally trailing the first cutting element, at substantially the same radial position over the bit face and having a superabrasive table oriented generally perpendicular to the profile of the bit face. The superabrasive table of the second cutting element may be carried on the outer end of a substrate configured as a stud-like carrier element over which the superabrasive table is formed and extends over the entire cross-section of the carrier element. It is preferable that the superabrasive table of the second cutting element exhibit a substantial thickness, a beveled, semifrustoconical rake face (at least facing in the direction of intended bit rotation) of considerable dimension, and a clearance face at a radially inner periphery of the rake face. The rake face may comprise a continuous, arcuate surface, or a series of laterally adjacent facets together simulating an arcuate surface.
In another embodiment of the invention, the second cutting element may be located at a position along the profile of the bit intermediate, or at least partially lying between, two first, relatively more aggressive, rotationally leading cutting elements.
The second cutting element is preferably slightly tilted with respect to a perpendicular to the profile of the bit face at the location of the second cutting element in a direction away from the intended direction of bit rotation so as to form a small clearance angle between the clearance face and the face of a formation being cut when the bit is drilling. Further, the second cutting element may be underexposed relative to its associated first cutting element; that is to say, the second cutting element protrudes from the bit profile a lesser distance than the first cutting element. In addition, the second cutting element may be side raked with respect to an associated first cutting element or elements.
Rotary drag bits including a plurality of cutting assemblies as described above are also within the scope of the present invention. Such bits may particularly feature such cutting assemblies on the shoulder region of the bit profile, although the invention is not so limited. It is contemplated that cutting assemblies of both of the foregoing configurations may be employed on the same drill bit. Stated another way, cutting assemblies comprising a single first cutting element and a single second cutting element may be employed on a bit in combination with cutting assemblies wherein two first, radially offset cutting elements have a second, at least partially radially intermediate cutting element associated therewith.
In various embodiments, the second cutting elements of the cutting assemblies of the invention provide significant protection against wear of the material of the bit body, and particularly on vertically, or axially, oriented portions of the bit body profile. If a first cutting element breaks, a trailing, second cutting element takes over to cut the formation. While performance may be diminished in such situations, the presence of the second cutting element prevents ring-out or groove-out of the bit body or blade on the profile, thus permitting replacement of the failed first cutting element when the bit is tripped from the well bore and rerunning of the bit. In addition, the placement and orientation of the second cutting elements promote enhanced bit stability even in situations where breakage of the first cutting element does not occur.
Other features and advantages of the present invention will become apparent to those of skill in the art through a consideration of the ensuing description, the accompanying drawings, and the appended claims.